1. Field of the Invention
The invention relates to a system and method for improving the wearability of surfaces that are exposed to wear conditions during their working life.
2. Background Art
The wear phenomenon is generally a physical form of material degradation in that it involves the removal of surface material as a result of mechanical and/or chemical action. Industry realizes that the amount of wear need not be significant before its manifestation becomes quite expensive. For instance, a car can be considered “worn out” following the loss of only a minor amount of material from critical surfaces that are in sliding contact.
It is known that there are various forms of wear: (1) adhesive wear—this occurs when two glissile surfaces slide over each other and debris is removed from one surface which adheres to the other; (2) abrasive wear—this happens when a hard, rough surface interacts with a softer surface, thereby forming indentations in the softer material and creating wear particles; (3) surface fatigue wear—this occurs during repeated sliding or rolling over a, for example, track; and (4) corrosive wear—this occurs with sliding in a corrosive environment, thereby augmenting the physical effects of wear with chemical degradation. All these forms of wear can be exacerbated by elevated temperature.
Cutting tools, because they experience most of the above noted forms of wear, to extreme levels and also at high temperatures, provide a good example of the challenges related to protecting a substrate from wear. Any manufacturer that works with machining materials is exposed to the problems of wear. This includes automotive, truck and engine OEMs and suppliers (cast aluminum, CGI), military equipment manufacturers (fiber reinforced composites including both metal- and polymer-matrix), among many smaller niche applications. Because certain materials are difficult to machine, cutting speeds are reduced. Various industries have recognized that significant manufacturing economies can be realized if the problems of wear can be avoided or ameliorated. Among other industries, the cutting tool industry is one example.
Modern cutting tool materials, including diamond (poly-crystalline (PCD) and thick film) and cubic-boron-nitride (CBN), are the hardest known and are becoming routinely used. However, their extremely high cost (compared to tungsten carbide, ceramics and cermets) limits their use in many cases, especially when the least expensive of these materials (tungsten carbide) can be enhanced with alloying and coatings (TiN, TiCN, Al2O3), thereby reducing its tendency to chemically dissolve into the chip and workpiece at high temperatures. Part of what makes all these substrate materials suitable for metal-cutting applications is their retention of hardness and abrasion resistance at highly elevated temperatures. These coatings, being harder than the substrate, also offer increased abrasion resistance, at least until worn through. Unfortunately, developments in these substrate materials and coatings are quite mature and may offer little more than incremental improvements in the near future.
One problem of prior approaches is the very high temperatures experienced by the cutting tool. Temperatures go up with increased productivity levels (i.e., cutting speed). Coatings have advanced from the basic single TiN CVD (chemical vapor deposition) coating to thinner PVD (physical vapor deposition) coatings and elaborate multi-layer CVD coatings with as many as nine or more alternating layers of different materials. See, e.g., U.S. Pat. No. 6,117,533. Coatings today serve as a hard, thermal barrier with good abrasion and chemical resistance compared to the substrate. However, it is generally known that typical (multilayer) CVD coatings are only 15-20 μm thick, with PVD coatings being only 2-3 μm thick. PVD coatings have to be thinner since their production process induces significant internal stress that causes flaking of thicker coatings under only small applied loads. CVD coatings, though much thicker, are limited in thickness due to the same internal stresses as for PVD coatings, as well as ones that develop due to the coefficient of thermal expansion mismatch across the large rake face surface and the delamination that occurs as the two sides of the interface expand at different rates. Either way, these coatings are thin, and often wear through rather quickly. They still provide resistance to chemical dissolution by dragging atoms of coating through the zone where the coating has worn through. However, their abrasion resistance is virtually gone once wear-through has occurred.
Accordingly, there is a need for an approach to maintain abrasion and chemical dissolution resistance for a significantly longer period of time, whether the application is cutting or one of many other wear surface scenarios. While diamond and CBN possess good abrasion/heat resistance, they are limited in their use (e.g., diamond cannot be used in contact with ferrous alloys due to a high propensity to dissolve in the iron), not to mention they are expensive relative to tungsten carbide. A fundamentally different coating approach may offer a major improvement over the current multilayering efforts and extend protection to more susceptible substrates (e.g., steel, aluminum, magnesium) in other harsh wear environments.
Since a partially worn through coating can sometimes still offer chemical/dissolution protection, a thicker coating would have its greatest impact in more abrasive environments. In the current example of cutting tools, specific work on CGI (Reuter and Schulz, 1999), metal-matrix composites (Hooper et al., 1999; Yanming and Zehua, 2000) and carbon/glass-fiber polymer composites (Gordon and Hillery, 2003) confirm the abrasive wear challenges in machining these materials. Machining of CGI is further hampered in continuous cutting due to its relatively high ductility (compared to gray iron), causing it to create higher temperatures on the cutting tool (Reuter and Schulz, 1999). Polymer-matrix composites along with metal-matrix composites (used significantly in modern aircraft and other lightweight applications) cause wear almost solely through abrasion (Yanming and Zehua, 2000). Since many of these materials are nonferrous, diamond coating in the manner disclosed below could be a huge improvement over current tooling including those with thin diamond films.
General hard-coating technologies include the processes of chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD coatings can be made thicker, but as a conformal deposition process, films can build up excessively on surface discontinuities and sharp curvature surfaces, especially in high pressure processing utilized to enhance the deposition rate. PVD coating processes, in several instances, are much faster than CVD processes and are able to form evenly despite strong surface curvature changes (Kvasnicka et al., 1999; Novak et al., 1999).
Several subcategories of chemical and physical vapor deposition processing exist. Plasma enhanced chemical vapor deposition (PECVD) is utilized for low temperature CVD processing, atmospheric pressure CVD (APCVD) is used for high deposition rate processing, and low pressure CVD (LPCVD) is extensively used as a low-rate, conformal, and high purity thin film process. PVD processes utilize the vaporization or physical redeposition of a source material onto a substrate. There is a large variety in the methods that make up PVD techniques, but those that allow for high levels of adhesion and good mechanical properties are typically deposited by plasma-based techniques. These include direct current (DC) and radio frequency (RF) sputter deposition, ion beam deposition, and cathodic arc deposition, among others. Some benefits of ion beam and cathodic arc deposition are enhanced deposition rates and high energies of the impinging ions resulting in very good adhesion and film properties (Popovic et al., 2004; Witke et al., 1999).
Another area where coatings are used is in microelectromechanical systems (MEMS). One relevant method is pulsed cathodic vacuum arc (PCVA) deposition of metals, alloys, and semiconductors, which has focused on the development of high-quality high deposition rate conductive silicon films for structural MEMS layers (Xia et al., 2004). The flexibility and capability of the PCVA technique allows for the reactive deposition of metallic oxide, carbide, and nitride compounds using metallic or semi-metallic deposition targets in oxygen-, methane-, or nitrogen-containing gas ambients. This PCVA technique, combined with a high chemical reactivity of the resulting ˜98% ionized metallic plasma, allows for the production of high quality and high density hard surface coatings. This technique has been applied extensively in volume production of TiN coatings in the automotive and machine tool industries. The capability of the technique applied to channels with high aspect ratios has been demonstrated for copper metallization at over 5:1 aspect ratios with void-free results (Siemroth and Schuelke, 2000).
As noted, MEMS applications have explored the coating and filling of micro-sized (i.e. below 1 millimeter) features. Such features are produced in non-silicon materials via micromachining. Micromachining focuses on the mechanical, electromechanical and laser processes used to fabricate small features in a wide variety of materials. It is differentiated from lithography whereby patterning is achieved by optical resists and wet chemistry, for example. Mechanical micromachining has grown over the past thirty years and is used in diverse industries such as medical implants and prostheses, semiconductor testing, automotive injection systems, optics, and micro-fluidic systems. Specialized machine tools, available in the marketplace, have closed-loop positioning systems with sub-nanometer resolution capable of machining micron features with sub-micron tolerances. Recent research has successfully machined (mechanically) free-standing structures with a lateral dimension of one micron as well as millimeter-scale mechanical parts with a tolerance of 0.25 microns (Friedrich and Vasile, 1996; Vasile et al., 1997), including micromilling of channels.
Micro electrical discharge machining (micro-EDM) uses a machined, or otherwise shaped, electrode immersed along with the workpiece in a dielectric fluid. Several control schemes are used to initiate an electrical spark in the gap between the shaped electrode and the workpiece. A shaped electrode for micro-EDM-ing of small channels is typically made of brass, which is easily micromachined. CNC control of micro-EDM allows untended batch production of parts. Laser ablation is another process that shows promise for large-scale surface microtexturing.